...with a T4 to get you 500bhp your not gonna be making full boost very early so you would want std comp to get the drivability before all hell breaks loose..i have had 7.1-1 and 7.9-1 on the same set up...to go std comp over low you would have to take extra precautions like use liners
(personnal preference) ..wrc long stud and wrc head gasket a nessescity IMO.
To lower the comp youd buy low comp pistons barring in mind how much your head is skimmed into the equasion.